This project proposes to investigate the role of the MHC in determination of the lifespan of Peromyscus individuals. The investigator proposes to select both a panel of inbred strains of Peromyscus as well as a panel of MHC congenic strains. These congenic strains will be selected for tissue incompatibility with the inbred partner strain. Further, (C57BL/6 x A/J)F2 mice will be analyzed to confirm previous suggestions that the H-2 complex of mice plays a partial role in determination of longevity.